Back in the '60s a commercial track where I raced was powered by a single standard automotive battery. An ordinary "12 volt" battery in good shape is 12.6 volts when it is fully charged (I just looked that up on Wikipedia). The charging voltage is 13.8 volts, so if you were to measure the battery in a car while it was running it would be more than 12.6 volts. A regular Scalextric "black stripe" motor will try to pull a bit over 2 amps when it starts up, the running amperage is much lower. I would expect that a full sized automobile battery would last all day if you were just running four Scalextric cars at a time. Chances are that the voltage would drop off gradually as time went by. You could minimize the drop-off by using a voltage regulator. As someone already mentioned it would be a good idea to fuse your track.